Preheat your oven to 350°F (175°C).
In a bowl, whisk together the flour, baking powder, and salt.
In a large bowl, cream the softened butter and granulated sugar until light and fluffy.
Add in the egg and vanilla extract, and mix until well combined.
Gradually add the dry ingredients to the wet mixture, mixing just until combined.
Add the green food coloring and mix until desired color is achieved.
If the dough seems too thick, add milk a tablespoon at a time until it reaches a workable consistency.
Chill the dough for at least 30 minutes to improve handling.
Roll out the dough to about ¼ inch thickness and cut out shamrock shapes.
Place the cookie shapes onto a baking sheet lined with parchment paper.
Bake for 8-10 minutes until the edges are lightly golden.
Allow cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack.
Once cooled, decorate with powdered sugar or an icing of your choice.